1. Field of the Invention
The present invention relates to photovoltaic modules. More specifically the present invention related to the protective backing sheets.
2. Description of Related Art
Solar energy utilized by photovoltaic modules is among the most promising alternatives to the fossil fuel that is being exhausted this century. However, production and installation of the photovoltaic modules remains an expensive process. Typical photovoltaic modules consist of glass or flexible transparent front sheet, solar cells, encapsulant, protective backing sheet, a protective seal which covers the edges of the module, and a perimeter frame made of aluminum which covers the seal. As illustrated in FIG. 1, a front sheet 10, backing sheet 20 and encapsulant 30 and 30′ are designed to protect array of cells 40 from weather agents, humidity, mechanical loads and impacts. Also, they provide electrical isolation for people's safety and loss of current. Protective backing sheets 20 are intended to improve the lifecycle and efficiency of the photovoltaic modules, thus reducing the cost per watt of the photovoltaic electricity. While the front sheet 10 and encapsulant 30 and 30′ must be transparent for high light transmission, the backing sheet must have high opacity for aesthetical purposes and high reflectivity for functional purposes. Light and thin solar cell modules are desirable for a number of reasons including weight reduction, especially for architectural (building integrated PV) and space applications, as well as military applications (incorporated into the soldier outfit, etc). Additionally light and thin modules contribute to cost reduction. Also reduction in quantity of consumed materials makes the technology “greener”, thus saving more natural resources.
One means to manufacture light and thin solar cells is to incorporate light and thin backing sheets. The backside covering material however, must also have some moisture resistance to prevent permeation of moisture vapor and water, which can cause rusting in underlying parts such as wires, and electrodes, and damage solar cells. In addition, backing sheets should provide electric isolation, mechanical protection, some UV stability, adherence to the encapsulant and ability to attach output leads.
Currently used protective backing sheets are typically laminates. FIG. 2 provides an illustration of a typical laminate backing sheet 20. The laminate consists of films of polyvinylfluorides 22, which is most commonly Tedlar®, polyesters (PET) 24, and copolymers of ethylene vinyl acetate (EVA) 26 as key components. The EVA layer 26 bonds with the encapsulant layer 30 in the module and serves as a dielectric layer and has good moisture barrier properties. It is dimensionally stable. White EVA allows significant power boost. The polyester layer 24 is very tough, has excellent dielectric properties, is dimensionally stable, and also has good moisture barrier properties. The polyvinylfluoride layer 22 serves as a very weatherable layer.
Even though these films have met performance standards in the required tests and during actual use, they exhibit certain limitations such as high cost and limited availability of the Tedlar® films. Another drawback of prior art materials such as PVF (Tedlar®), ECTFE (Halar®) and other fluoropolymers, is that such materials cannot be processed at ambient or moderately elevated temperatures. For example, PVF film is produced by a casting process from dispersion, using high boiling solvents (usually dimethyl acetamide for oriented Tedlar® and propylene carbonate for cast Tedlar®). The boiling point of dimethyl acetamide is 164-166° C. and the boiling point of propylene carbonate is 200° C. The dispersion must be processed at 160° C. and 90% of solvent content or greater to ensure adequate film formation. Higher temperatures are unacceptable due to PVF resin thermal instability: its fusion and decomposition temperatures are so close, that PVF can decompose during the baking. As a result, there is always a residual solvent in Tedlar® film. DuPont reports that residual amounts of dimethyl acetamide (DMAC) ranging from 0.05 to 1.0 wt % will be present in all oriented Tedlar® PVF films.
Alternatively, ECTFE (Halar®) films are produced by melt extrusion at 350° C.-375° C. As a result, they cannot be easily compounded with pigments, clays, etc. and are also expensive.
U.S. Pat. No. 5,741,370 suggests that manufacturing and module mounting costs could be reduced by using, as the backskin material, a thermoplastic olefin comprising a combination of two different ionomers, e.g., a sodium ionomer and a zinc ionomer, with that combination being described as producing a synergistic effect which improves the water vapor barrier property of the backskin material over and above the barrier property of either of the individual ionomer components. Also, the patent discloses use of an ionomer encapsulant with the dual ionomer backskin.
However, National Renewable Energy Laboratory (NREL) reports that ionomer resins contain free and bound methacrylic acid, which requires using stainless steel tooling during melt processing, thus increasing the manufacturing costs. PVMaT Improvements in the Solarex Photovoltaic Module Manufacturing Technology Annual Subcontract Report May 5, 1998-Apr. 30, 1999, National Renewable Energy Laboratory, January 2000 • NREL/SR-520-27643.